A bounded linear operator T on a Banach space is said to be dissipative if e tT 1 for all t 0. We show that if T is a dissipative operator on a Banach space, then: (a) lim t→∞ e tT T = sup{|λ|: λ ∈ σ (T ) ∩ iR}. (b) If σ (T ) ∩ iR is contained in [−iπ/2, iπ/2], then lim t→∞ e tT sin T = sup | sin λ|: λ ∈ σ (T ) ∩ iR .Some related problems are also discussed.